Method for determining tribological properties of a sample surface using a scanning microscope (sem) and associated scanning microscope

ABSTRACT

A method for examining a surface of a sample is described using an atomic force scanning microscope (AFM) comprising a cantilever with a longitudinal extension along which a measuring tip is disposed, which is selectively arranged relative to the sample surface by a means for driving and whose spatial position is detected using a sensor unit. Vibration excitation is conducted at excitation amplitudes which produce inside the cantilever torsional amplitudes with maximum values which form a largely (substantively) constant plateau value despite increasing excitation amplitudes and the resonance spectra, in a range of maximum values of the torsional amplitudes, a widening of the resonance spectrum which is determinable by a plateau width. The resonance spectra, preferably the plateau value, the plateau width and/or the gradient of the respective resonance spectra are used for examining the sample.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for examining a sample surfaceusing an atomic force scanning microscope comprising a cantilever with alongitudinal extension, along which the measuring tip is arrangedprecisely relative to a sample surface by means of a means for driving,the spatial position of the measuring tip being determined by a sensorunit. The microscope is further provided with at least one ultrasoundgenerator with which vibration excitation is initiated at a givenexcitation frequency between the sample surface and the cantilever. Themeasuring tip of the cantilever is brought into contact with the samplesurface in such a manner that the oscillations imparted to the measuringtip are oriented lateral to the sample surface and perpendicular to thelength of the cantilever. The torsional vibrations induced in thecantilever are detected and analyzed by means of an evaluation unit.

2. Description of the Prior Art

The development of an atomic force scanning microscope has permittedmajor achievements in the field of examination of surface properties, inparticular in the characterization of surface properties. For the firsttime, it is possible to obtain information concerning surfaces and areasclose to the surface of very different samples in nanometer resolutioneven in the magnitude of single atoms. Friction force microscopy, afurther development of the atomic force scanning microscope, permittedfor the first time studying one of the oldest problems in science, theexamination of friction, on this scale.

DE 43 24 983 C2 describes an acoustical microscope operating on thetechnological basis of an atomic force scanning microscope that is ableto measure the topography as well as the elastic properties of thesurface of a sample. The microscope comprises a cantilever designed as aleaf spring, usually with a length of between 100 μm and 500 μm,attached to the one end of which is a pyramid-shaped measuring tiphaving a tip radius of curvature of about 50 nanometers.

In order to measure and examine the sample surface holistically, thecantilever and the measuring tip attached thereto are scanned over thesample surface with the aid of a suitable means for moving in such amanner that the measuring tip makes contact with the sample surface witha given vertical load at every single scanning point. The optical sensorunit permits determining the degree of deformation of the cantilever andthus the topography-based excursion of the measuring tip. Usually, theoptical sensor unit is provided with a laser diode from which a laserbeam directed at the cantilever is emitted, reflected thereat, anddetected by a position-sensitive photodiode. During scanning, thecantilever and the measuring tip are guided perpendicular to the samplesurface via a regulation loop in such an active manner that theexcursion of the cantilever, and the vertical load with which thecantilever lies on the sample surface via the measuring tip, remainsconstant. The regulation tension required for the excursion is usuallyconverted into a distance value and is correspondingly depicted as anencoded color value in a representation showing the surface topography.

In order to also be able to determine the elastic properties of thesurface sample, an ultrasound generator is provided which inducesoscillations in the surface sample while the measuring tip lies at ascanning point of the sample surface. Vibration excitation by couplingin ultrasonic waves leads to normal vibrations of the sample surfacewhich induce high-frequency oscillating bending vibrations in thecantilever along its longitudinal extension.

Detection by the ultrasonically induced, high-frequency vibrationbehavior of the cantilever permits obtaining information about theelastic properties of the sample surface. The problem with thismeasuring situation that needs to be resolved lies in the decoupling dueto the measurement of the superimposed excursions of the cantilever,which result, on the one hand, from the topography measurement due towhich the vertical load with which the measuring tip lies on the samplesurface remains, as constant as possible and, on the other hand, whichcause the ultrasonically induced normal vibrations of the sample surfacetransmitted to the cantilever via the measuring tip.

In order to obtain a reliable measuring signal with a high as possiblesignal/noise ratio for measuring the elasticity, the ultrasonicallyinduced vibration excitation of the sample surface occurs at frequencieswhich are at least one magnitude greater than the resonant frequency ofthe cantilever having the measuring tip attached thereto. Using twophotodiodes with different temporal responding behavior, on which thelight beam reflected at the cantilever impinges, permits selectivedetection and evaluation of the vibration behavior of the cantilever.Thus, the photodiode with a slow response behavior is able to solelydetect the excursions resulting from the contour-based readjustment ofthe cantilever for determining the topography. On the other hand, thesecond photodiode, which has a bandwidth window in the MHz range, isprovided for determining the high-frequency vibration parts of thecantilever. Especially suited therefor are, for example, single-celllight-sensitive detectors with a smooth-edged means for shading, forexample in the form of a razor blade or a so-called heterodynerunning-time interferometer, in the one interferometer arm of which afrequency shift means is provided. Such a rapid type respondingdetection unit can also be designed based on a capacity measurement, inwhich the measuring capacity is formed from the cantilever and aneedle-shaped counter-electrode disposed opposite thereto. Furtherdetails can be found in the aforementioned printed publication DE 43 24983 C2.

Contrary to the aforedescribed resonance measurement with verticalmodulation, that is the to-be-examined sample surface is excited tonormal vibrations U.S. Pat. No. 5,804,708 describes an atomic forcemicroscope with a similar setup, but vibration excitation of theto-be-examined sample occurs with the aid of a signal generator in sucha manner that the sample surface imparts vibrations oriented lateral tothe sample surface and, in particular, directed transverse in relationto the longitudinal extension of the cantilever.

The vibration excitation directed transverse to the longitudinalextension of the cantilever induces torsional vibrations in thecantilever in contact with the sample surface via the measuring tip,with the measuring tip, which is at least sometimes in contact with thesample surface, executing oscillations which are directed inlongitudinal direction to the sample surface and transverse to thelongitudinal extension of the cantilever, respectively are polarized.The measuring tip briefly adheres to the sample surface at the point ofreversal of the oscillations. The sample surface is deformed by theshear forces acting laterally to the sample surface until, due tofriction, the measuring tip slips out of the described state back overthe sample surface.

The shear deformations formed at the returning points in dependence onthe vertical load with which the measuring tip lies on the samplesurface influence the vibration behavior of the measuring tip andconsequently that of the cantilever in a manner which characterizes theelastic properties of the sample surface. Thus, it is possible to obtaininformation about the elastic properties of the sample surface from thevibration behavior, for example from the vibration amplitude and/or thephase of the oscillations occurring in the form of torsional vibrationsalong the cantilever.

The oscillations initiated by the signal generator in the sample havefrequencies of approximately 1 kHz. However, with this measuring method,local resolution has proven unsatisfactory. Only measurements with alocal resolution of approximately 100 nm can be achieved. Moreover, themeasuring quality achievable with this method permits obtaining onlyqualitative information about the frictional properties of the samplesurface.

SUMMARY OF THE INVENTION

The present invention is a method for examining a surface sample usingan atomic force scanning microscope of the aforedescribed manner, inwhich vibrations are induced in the surface sample, the vibrations beingdirected lateral to the sample surface and, moreover, being orientedperpendicular to the longitudinal extension of the cantilever, in such amanner that it is possible to obtain qualitative and quantitativeinformation about the frictional properties of the sample surface. Inparticular, method permits high locally resolved determination of thetribological, that is frictional properties of the sample surface, bymeans of superimposing a topography measurement, permitting in thismanner finely as possibly resolved sample surface mapping with a localresolution of less than 100 nm, preferably less than 10 nm.

A key element of the present invention is that a method for examining asample surface by means of an atomic force scanning microscopecomprising a cantilever with a longitudinal extension, along which ameasuring tip is disposed, which is selectively arranged relative to thesample surface via a means for driving and the spatial position of whichis detected by a sensor unit, and is provided with at least oneultrasound generator, which initiates a vibration excitation with agiven excitation frequency between the sample surface and thecantilever. The measuring tip of the cantilever is brought into contactwith the sample surface, in such a manner that the vibrations impartedto the measuring tip are oriented lateral to the sample surface andperpendicular to the longitudinal extension of the cantilever. Torsionalvibrations that are formed in the cantilever are detected and analyzedby means of an evaluation unit. Vibration excitation occurs in such amanner that the oscillations executed by the measuring tip have higherharmonic vibration parts relative to the excitation frequency. Thevibration excitation preferably occurs with a continuous wave signalwhich is wobbled, that is varied, within a given excitation frequencyrange. The excitation frequency range is selected in such a manner thatthe resonant basic vibration of the cantilever having the measuring tipin contact on the sample surface lies inside the excitation frequencyrange.

In addition to the selection of a suitable frequency, vibrationexcitation of the cantilever lying on the sample surface occurs withexcitation amplitudes causing in the cantilever torsional vibrationswith torsional amplitudes whose torsional amplitude maximum valuesassume a largely constant plateau value despite increasing excitationamplitudes and whose resonance spectra undergo in the range of thetorsional amplitude maximum value a widening of the resonance spectrumwhich is determinable by the width of the plateau. Finally, theresonance spectra, preferably the plateau value, the plateau width, thegradient of the respective resonance spectra at the flanks of theresonance curve and/or the gradient of the plateau can be utilized toexamine the sample surface.

With the aid of the method of the invention, tribological properties,thus for example, the frictional forces or the frictional coefficientsacting between the measuring tip and the sample surface, are detected atthe sample surface with a local resolution of up to 1 nm. Compared toprior art methods, which at best permit local resolution ofapproximately 100 nm, the method of the invention is a highly sensitiveand most finely suited to a resolving tribological method of analysis.In addition to determining tribological properties at a sample surface,the method of the invention, of course, also permits determination ofthe topography by adjusting a constant vertical load which the measuringtip of the cantilever lies on the to-be-examined sample surface. Withthe aid of a means for detecting, low-frequency excursions of themeasuring tip are detected via the reflection of light at the cantileverand correspondingly evaluated. The detection signal obtained with themeans for detecting representing the low-frequency topography-basedexcursion of the measuring tip serves, on the one hand, to determine thetopography and, on the other hand, as a regulation value, with which thedistance between the measuring tip and the sample surface, and thevertical load with which the measuring tip lies on the sample surface isheld constant and temporally averaged. In this manner, the method of theinvention permits rendering in successive scanning of the surface anaccurate microscopic topographic image of the sample surface in a scaleof up to 1 nm, the image being able at the same time to providetribological information about the sample surface.

Measurement of tribological surface properties at a point of the samplesurface preferably occurs in several steps. First, for determining thebasic resonant frequency of the cantilever in contact with the samplesurface via the measuring tip, the ultrasound generator generatesvibrations in the form of continuous wave signals whose frequencies arewobbled, that is varied, in a given frequency range. The given frequencycomprises preferably frequencies below the basic resonant frequencyrange of the cantilever in contact with the sample surface via themeasuring tip up to thirty times this contact resonant frequency.Typically, frequency wobbling of the excitation frequency occurs in 1kHz frequency steps within a frequency range between 50 kHz and 10 MHz.For example, in the case of a typical cantilever with a length of 450μm, there were four torsional resonances in the frequency range between50 kHz and 3 MHz.

In order to determine the properties of the sample surface, inparticular with regard to the tribological properties, such asfrictional coefficients etc., the sample is impinged via the ultrasoundgenerator with excitation frequencies lying in the contact resonantfrequency f_(r). Preferably, the excitation frequency range comprisesΔf_(a) frequencies from f_(r)−½f_(r) to f_(r)+½f_(r). In a particularlyadvantageous manner, the excitation frequency range Δf_(a) comprisesfrequencies between f_(r)−½Δf_(r) to f_(r)+½Δf_(r), with Δf_(r)corresponding to the half-width value of the determined resonance curvemeasured at f_(r).

Vibration excitation occurs within the framework of a frequency sweep,that is the excitation frequency is wobbled, and varied, in the givenexcitation frequency range Δf_(a) in the form of single continuous wavesignals.

In addition to selecting the excitation frequency range in the range ofthe contact resonant frequency, of utmost importance is the exactsetting of the direction of the vibrations, respectively of thepolarization of the vibration of the transverse vibrations inducedlaterally in the sample surface relative to the longitudinal extensionof the cantilever. Setting the measuring tip lying on the sample surfacewith a defined vertical load results in high-frequency oscillatingtransverse vibrations transverse to the longitudinal extension of thecantilever which due to the great rise in resonant vibrations constantly“jumps back and forth” between the following three states: 1) themeasuring tip rubs over the sample surface; 2) oscillation movementcomes to a standstill; 3) the measuring tip moves within an elasticpotential, that is the measuring tip briefly engages in a frictionalbond with the sample surface, locally deforming the sample surface dueto the shear forces directed lateral thereto.

In contrast to the non-resonant case, as described in U.S. Pat. No.5,804,708, in which the measuring tip executes strict cyclicaloscillations with the measuring tip, in the resonant vibration case, themeasuring tip dances at least sectionwise chaotically over the samplesurface and assumes the aforedescribed states stochastically. This isreferred to as “stick-slip” motion. This motion represents a highlydynamic motion behavior.

Due to the measuring situation described above, it is not difficult tounderstand that the vibration behavior forming inside the cantilever isdetermined by the tribological contact properties between the measuringtip and the sample surface. If the sample surface is excited, asmentioned in the preceding, with a contact resonant frequency,preferably the basic resonant frequency of the cantilever in contactwith the sample surface via the measuring tip to vibrations by means ofthe ultrasound generator, at low excitation amplitudes, resonantvibration behavior of the cantilever sets in, the resonance curve ofwhich is largely symmetrical. The resonant vibration behavior of thecantilever is detected in a prior art manner by means of an opticalsensor unit and is represented in the form of a resonance curve. If theexcitation amplitude is raised by successively increasing the excitationvoltage with which the ultrasonic wave generator is operated, therecorded resonance spectrum shows deviations from the originallysymmetrical resonance curve of such a manner that, despite increasingexcitation amplitude, the amplitude of the resonance spectrum assumes atype of saturation value and remains practically constant. Similarly,the form of the resonance curve changes in such a manner that a wideningis generated in the upper amplitude range or the resonance curve. Alongwith the widening of the resonance spectrum of the resonance curve, asort of plateau forms, whose position remains largely constant despiterising excitation amplitudes, the width of which however also increaseswith rising excitation amplitudes. According to the invention, it isthese characteristic deviations from the symmetric formation of theresonance curve formed by the increase in excitation amplitude that areselectively used to obtain tribological information. This particularlyapplies to the plateau values, the plateau width, the gradient of therespective resonance spectra at the flanks of the resonance curve and/orthe gradient of the plateau yielded by a widening of the resonancespectrum.

The aforedescribed resonant excitation can, of course, also be carriedout at contact resonant frequencies of a higher order. Thus, theaforedescribed deviations from the symmetric formation of the resonancecurve can be observed not only at the basic resonant frequency, that isin the occurrence of the first torsion mode, but also at higher modes.The widening occurring in the resonance curve at higher modes, such asin the plateau width, can also be utilized for determining thefrictional force.

In addition, “overtones” to the excitation frequency can be detected inthe resonant behavior of the cantilever as soon as the describedflattening at the resonance maximum sets in. Such type overtones canalso be detected at higher vibration modes, which also can be utilizedto determine the frictional force. For example, if a cantilever incontact with the surface has the first torsion mode at an excitationfrequency of 100 kHz, the higher torsion modes lie at 300 kHz, 500 kHz,700 kHz, etc. The n^(th) torsion mode, therefore, lies at (2n−1)×100kHz. If the first torsion mode is excited with a sufficiently highexcitation amplitude so that, for example, a flattened torsion peak isvisible in the excitation frequency spectrum between 80 kHz and 120 kHz,peaks occur as well at 200 kHz, 300 kHz, 400 kHz, etc., also atfrequencies k×100 kHz, which are singly detectable. The overtones of theexcitation frequency that coincide with higher torsion modes (300 kHz,500 kHz, 700 kHz, . . . ) are, of course, more intensive than the others(200 kHz, 400 kHz, 600 kHz, . . . ).

For detection of the resonant torsional vibrations forming inside thecantilever, at least one temporally high-resolving photodiode is used,whose temporal resolution capacity permits detection of the vibrationoccurrences with frequencies, which preferably correspond to up totwenty-five times, preferably double to ten times the excitationfrequency.

By means of the sequential scanning in the measuring tip along thesample surface, measurements are conducted successively at closelyadjacent contact points spaced laterally at least approximately 1 nmunder the aforedescribed resonance conditions. On the one hand, themeasurements yield information about the surface topography as well as,on the other hand, about the tribological properties at the point ofcontact. In addition to the topographically determined surface contour,the aforementioned properties of the resonance curve of the cantileverat each point of the to-be-measured sample surface can be plotted andencoded as a color value for representation. For example, changingfrictional properties at the sample surface influence the resonantvibration behavior of the cantilever and therefore the vibrationamplitude at constant excitation frequency making even the smallestchanges in friction detectable as it is these smallest changes infriction that have very sensitive influence on the amplitude behavior,as is clearly indicated by the recorded resonance curves.

For example, the smallest shifting of the flanks of the resonance curvein relation to the frequency axis (x-axis) results in major changes inthe resonance amplitude (y-axis). As already mentioned, besidesdetecting the resonant behavior of the basic vibration of thecantilever, higher harmonic resonances can also be detected and examinedand correspondingly encoded as a color value for representation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is made more apparent in the following usingpreferred embodiments with reference to the accompanying drawings, byway of example, without intention of limiting the scope or spirit of theinvention.

FIG. 1 shows a schematic representation of components for conducting theinvented method; and

FIG. 2 shows a diagrammatic representation with resonance curves atdifferent excitation amplitudes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an atomic force scanning microscope for conducting themethod of the invention for examining a sample surface, in particularfor determining tribological properties on the sample surface. Themicroscope depicted in FIG. 1 is provided with a cantilever 1, whosemeasuring tip 2 lies on the sample surface 3 of a sample P. Sample P isin contact with an ultrasonic transducer 5 via a pre-run track orpre-run layer 4. The ultrasonic transducer 5 is set into oscillations bya corresponding signal generator 6. The pre-run layer 4 is, for example,connected on both sides to the sample P and the ultrasonic transducervia a honeycomb layer as an acoustic coupling layer.

An optical sensor unit comprising a laser diode 7, a deflection mirror 8and a photodiode unit 9, is provided for measuring the vibrationsconveyed into the cantilever 1 via the measuring tip 2. The photodiodeunit 9 serves, on the one hand, to detect the topographically based,low-frequency excursions of the measuring tip 2 and therewith of thecantilever and, for this reason, is connected to an AFM back-couplingloop 10, which serves to constantly adjust the vertical load with whichthe measuring tip 2 lies on the sample surface 3. Details concerningsuch a type control loop are described in the printed publication DE 4324 983 C2 described in the introductory part hereof.

Similarly, the photodiode unit 9 is able to detect high-frequencyvibration parts which are conveyed as a torsional signal T to acomputing unit 12 which may perform spectral analysis via a rapid signalprocessing unit 11 which may be associated with a wideband amplifier,stored, evaluated and finally graphically represented as frictionalproperties.

For reason of clarity, the friction microscope setup shown quiteschematically in FIG. 1 does not show the means for driving required forthe spatial arrangement of the cantilever relative to the samplesurface, usually provided as a piezo driver means. As it is astate-of-the-art means for driving, here too reference is made to DE 4324 983 C2.

In order to carry out the examination on sample P according to thepresent invention, the object of which is measuring the tribologicalproperties at sample surface 3, the ultrasonic transducer 5 is designedand operated in such manner that sample P is set in vibrations solelylateral to the sample surface 3. The vibrations are, in addition,oriented perpendicular to the longitudinal extension of the cantilever1, respectively are polarized (see arrow in FIG. 1). The mechanicalcoupling sets the cantilever 1 in contact with the sample surface 3 viathe measuring tip 2 in torsional vibrations, which upon reaching a basicresonant frequency lead to a great rise in torsional resonancevibration. For selective determination of the basic resonant frequencyf_(r) of the cantilever 1 in contact with the sample surface 3 via themeasuring tip 2, the ultrasound generator 5, which is composed of thevibration generator 6 and the ultrasonic transducer 5, generates amultiplicity of continuous wave signals separated in temporalsuccession, whose excitation frequencies are wobbled in a givenfrequency range, including frequencies below the basic resonantfrequency of the cantilever up to thirty times this frequency, therebyensuring that cantilever 1 is excited to torsional vibrations not onlywith its basic vibration but also begins vibrating at higher modetorsional resonances. Upon reaching a contact resonant frequency, eitherthe basic resonant frequency or a higher harmonic resonant frequency,the excitation amplitude, with which the ultrasonic transducer 5vibrates, is set in such a manner that measuring tip 2 rubs on thesample surface 3, thus always changing the elastic contact to the samplesurface. In detail, at these excitation amplitudes, the measuring tip 2carries out oscillating sliding movements which are briefly interruptedat the point of reversal of the oscillation by friction bonding betweenthe measuring tip 2 and the sample surface 3.

The resonance behavior of the cantilever 1 setting in with thisvibration behavior, also described as “stick-slip” vibration behavior,is detected by an optical sensor unit 9 and analyzed more exactly by wayof a resonance curve representation. A family of curves obtained withthe aid of the measuring setup described in FIG. 1 is depicted in adiagram shown in FIG. 2, which provides an abscissa formed as afrequency axis and an ordinate formed as an amplitude axis. Theresonance curves depicted with the different sorts of lines representthe resonance behavior of the cantilever at different excitationamplitudes, and excitation voltages. It appears that at low vibrationamplitudes of the ultrasonic transducer, the amplitudes of therespective resonance maximum values increase linearly with the amplitudeof the excitation vibration. At excitation voltages of approximately upto 3 to 4V, largely symmetrical resonance curves form. From a certainexcitation amplitude, and excitation voltage, the amplitudes of theresonance curves, respectively of the torsion resonances, remain largelyconstant despite rising excitation voltages, but rather the shape of theresonance curve changes. The reason for such type nonlinear changes inthe shape of the resonance curve is found in the aforedescribed“stick-slip” behavior. If however the excitation amplitude is raisednonetheless, the diagram shows that the position of the torsionalresonance remains largely the same and a widening of the spectrum in theform of a plateau occurs in the range of the torsional maximum. It isthese curve-changing characteristics that are utilized according to thepresent invention to determine the frictional properties, and thetribological properties, of the sample surface. This relates, inparticular, to the plateau value of the resonance amplitudes, theplateau widths and the gradient of the resonance curve flanks forming asaturation value.

The evaluation of the resonant torsional vibration behavior of thecantilever occurs by means of recording the phase distribution andfrequency distribution of the torsional vibrations of the cantilever byway of optical determination of vibrations including using a lock-inamplifier. An alternative to the lock-in amplifier is using a widebandamplifier in conjunction with discrete signal processing for spectralanalyses, such as for example the discrete Fourier transformation (DFT),the rapid Fourier transformation (FFT), the wavelet transformations, orthe so-called Walsh transformation. Analog spectral analysis is alsofeasible.

LIST OF REFERENCE

-   cantilever-   measuring tip-   sample surface-   pre-run layer-   ultrasonic transducer-   signal generator-   laser diode-   deflection mirror-   photodiode unit-   AFM back coupling loop-   rapid signal processing unit-   computing unit-   sample-   torsional signal-   excitation frequency-   amplitude

1. A method for examining a sample surface using an atomic forcescanning microscope comprising a cantilever with a longitudinalextension along which a measuring tip is disposed, which is locatedrelative to said sample surface by a means for driving and having aspatial position detected with a sensor, and at least one ultrasoundgenerator, which initiates vibration excitation at an excitationfrequency between said sample surface and said cantilever, the measuringtip being brought into contact with said sample surface so that saidmeasuring tip is excited to vibrations oriented lateral to said samplesurface and perpendicular to said longitudinal extension of saidcantilever, torsional vibrations being induced in said cantilever whichare detected and analyzed by an evaluation unit, said vibrationexcitation causing oscillations of said measuring tip including harmonicvibrations relative to the excitation frequency and said vibrationexcitation includes excitation amplitudes which cause torsionalamplitudes within the cantilever with maximum values thereof forming aplateau of resonance spectra despite increasing excitation amplitudesand the resonance spectra which undergoes, in a range of said maximumvalues of said torsional amplitudes, a widening which is determinable bya plateau width, comprising: using at least one of the plateau of saidresonance spectra, a width of the plateau of said resonance spectraand/or a gradient of said resonance spectra for examining said samplesurface.
 2. The method according to claim 1, wherein: sequentialscanning at a multiplicity of different points of contact between saidmeasuring tip and said sample surface successive resonance spectra aredetected and analyzed.
 3. The method according to claim 2, wherein:information obtainable from said resonance curve at each point ofcontact between said measuring tip and said sample surface comprises atleast one of a half-width value Δf_(r) of said resonance curve at f_(r)wherein f_(r) is the excitation frequency, a plateau width, a plateauvalue, a gradient at said plateau or a vibration amplitude of harmonicsare recorded and represented as encoded color values.
 4. The methodaccording to claim 1, wherein: tribological properties are analyzed andqualitatively and/or quantitatively determined.
 5. The method accordingto claim 4, wherein: the tribological properties comprise a frictionalforce and/or frictional coefficients at said sample surface.
 6. Themethod according to one of the claim 1, wherein: said measuring tipmakes contact on said sample surface with a vertical load which isconstantly adjusted by said means for driving.
 7. The method accordingto claim 1, wherein: said ultrasound generator emits a continuous wavesignal vibrating at said excitation frequency with said continuous wavesignal being varied by means of frequency wobbulation within a givenexcitation frequency range.
 8. The method according to claim 7, wherein:said excitation frequency range is selected such that the resonantvibration of said cantilever in contact with said sample surface viasaid measuring tip is contained within said frequency range.
 9. Themethod according to claim 8, wherein: said sample surface is impingedwith a frequency sweep for determining the resonant vibration of saidcantilever lying on said sample surface with said measuring tip.
 10. Themethod according to claim 9, wherein: said frequency sweep comprises thefollowing range of frequencies f:f<f_(r) and f<30f_(r). where fr is a resonant frequency.
 11. The methodaccording to claim 7, wherein: said excitation frequency range comprisesfrequencies ranging from f_(r)−½f_(r) to f_(r)+½f_(r), corresponding toa half-width value of a resonance curve at f_(r).
 12. The methodaccording to claim 11, wherein: said frequency range comprisesf_(r)−½Δf_(r) to f_(r)+½Δf_(r), with Δf_(r) corresponding to ahalf-width value of the resonance curve at f_(r).
 13. The methodaccording to claim 7, wherein: said torsional vibrations of saidcantilever lying on said sample surface with said measuring tip aredetected using said sensor unit at a frequency range n Δf_(a), withn<25, wherein Δf_(a) is the excitation frequency range.
 14. The methodaccording to claim 13 wherein 2≦n≦10.
 15. The method according to claim1, wherein: said vibration excitation of said sample surface is causedby said ultrasound generator so that said ultrasound generator isdirectly or indirectly acoustically connected with said sample surface.16. The method according to claim 1, wherein: a microscopic image ofsaid sample surface is obtained by means of sequentially scanning saidsample surface which said microscopic image containing informationrelating to a surface topography and tribological properties.
 17. Themethod according to claim 1, wherein: said torsional vibrations insidesaid cantilever are detected by said sensor unit and sensor signalsobtained by said sensor unit are examined with a wideband amplifierfollowed by spectral analysis.
 18. The method according to claim 17,wherein: said spectral analysis is conducted using numerical Fouriertransformation or FFT, Wavelet-transformation or Walsh-transformation.